Radio link monitoring is vital to maintain radio connections in cellular systems. By regularly reporting the radio conditions to the system different types of actions can be taken when radio link failure occur.
In UTRA, the physical layers estimate the quality of the radio links and reports on radio frame basis to higher layers the synchronization status via so called synchronization primitives, as described at 3GPP TS 25.214 V7.4.0 “Physical layer procedures (FDD)”.
Reporting radio link quality status will also be needed in E-UTRA, in which a fast and reliable detection of radio problems would be beneficial in order to avoid unnecessary interference in uplink, waste of resources in downlink and unnecessarily long delays before cell reselection or handover can take place.
Radio Link Failure Handling in E-UTRA
The handling of radio link failure in E-UTRA is described in 3GPP TS 36.300 V8.1.0 “Overall description; stage 2” and consists of two phases as illustrated in FIG. 1, briefly described as
First Phase:
                It is started upon radio problem detection which may lead to radio link failure detection after e.g. that a timer period has expired (T1)        Network based mobility handlingSecond Phase:        It is started upon radio link failure detection which may lead to RRC state transition from CONNECTED to IDLE after timer (T2)        UE-based mobility handling        
The judgment of “radio problem detection”, as well as procedures for its reporting, is handled by the physical layer. The analogy with UTRA refers to the use of the synchronization primitives, e.g. the “out-of-sync” handling, which is further discussed below.
Radio Problem Detection in UTRA
In UTRA, fast transmit power control (TPC) is crucial for the system to operate properly. In downlink for example, TPC commands are sent continuously or periodically via the physical control channel (F-)DPCH or DPCCH. The UE measures the quality of this physical control channel over a pre-specified time interval and if the quality is below a specified threshold, the UE reports “out-of-sync” to the network. Error detection through cyclic redundancy check (CRC) is provided to transport blocks mapped to DPDCH. In addition to monitoring the control channel quality, the UE also monitors the number of consecutive CRC failures and if it is above a pre-specified number the UE reports out-of-synch to the network.
Packet Data Transmission in E-UTRA
E-UTRA is a pure packet data designed cellular system, in which transmissions of user data in uplink and downlink always take place via shared channels. Orthogonal Frequency Division Multiple (OFDM) technology is used in the downlink, whereas DFT based pre-coded OFDM is used in uplink. As similar to HSPA in UTRA, the UE monitors physical downlink control channels (PDCCH) in order to access UE dedicated user data on the physical downlink shared channel (PDSCH) and the network assign uplink scheduling grants to the UE on demand basis for uplink transmission via the physical uplink control channel (PUCCH) and the physical uplink shared channel (PUSCH). Error detection is provided on transport blocks and control payloads through CRC, and HARQ operations ensure efficient re-transmissions.
In E-UTRA, no downlink transmit power control (TPC) has been specified and uplink TPC commands are embedded in the control payload mapped to PDCCH, which are sent occasionally by the E-UTRA base station (eNodeB). Thus, no continuous or periodic dedicated physical channels such as (F-)DPCH and DPCCH are present as in UTRA.
Downlink Physical Signals and Channels in E-UTRA
The physical layer signals and channels in E-UTRA downlink are:                Physical layer signals, i.e. reference signal (pilots) and synchronization signals        Physical broadcast channel (PBCH)        PDCCH and PDSCH        Physical control format indicator channel (PCFICH)        Physical HARQ indicator channel (PHICH)        
Following observations can be done:                Physical layer signals and PBCH are transmitted periodically        Error detection via CRC of transport blocks mapped to PBCH and PDSCH, and of control data mapped to PDCCH        Some uplink transmissions shall result in downlink responses through the physical channels PDCCH and PHICH        
An advantage of considering periodically transmitted signals and channels that is that the reporting instants of radio problem detection to higher layers then can be known in advance. Using shared channels for monitoring the radio link quality in E-UTRA may lead to an unpredictable reporting delay due to absence of scheduled data.
Radio Problem Detection in E-UTRA Downlink
For E-UTRA downlink operations, it seems natural to characterize radio problem detection as a UE not able to detect PDCCH and/or PDSCH under a certain time period. Besides not being able to receive any user data, consecutive CRC failures of the control data would also imply that the UE cannot receive any uplink scheduling grants as well as not respond to uplink TPC and time alignment (TA) commands, which are vital for the system to operate properly.
However, for the PDCCH the UE ID is implicitly encoded into the CRC which implies that a CRC failure may not be due to bad radio link conditions, i.e. the packet was aimed for another UE. For that reasoning, judge radio problem detection on e.g. high number of consecutive CRC failures of control data mapped to PDCCH appears to be less useful.
Checking the CRC of transport blocks mapped to PDSCH could be one possibility to monitor radio problem detection. Although demodulation of PDSCH requires that the associated PDCCH was correctly detected, the PDCCH should be significantly more robust than PDSCH. In this case, the rate adaptation has resulted in lowest possible user data bit rates but still the UE responds with consecutive NACKs to eNodeB. However, a potential drawback to base radio problem detection only on e.g. a high number of consecutive CRC failures of transport blocks of PDSCH is that the absence of scheduled data will lead to an unpredictable and possibly very long reporting delay.
In E-UTRA, the common reference signals are sent periodically and are distributed over the whole system bandwidth. Monitoring some quality metric such as e.g. SIR or pilot symbol error rates of the reference signals and report radio problem detection when quality metric is above a certain threshold could be an alternative or an addition to the above PDSCH CRC checks. However, bad quality of the common reference signals does not necessarily reflect the perceived quality of downlink physical channels PDCCH and PDSCH. Additionally, a quality metric can also be associated with synchronization signals such as e.g. the correlations between receive synchronization signal and considered primary synchronization signal are below a certain threshold.
Error detection through CRC is also provided on transport blocks mapped to the PBCH, which (in contrast to transport blocks mapped to the shared channels) are sent periodically and thus will have a predictable reporting delay. Thus, consecutive failures to read the physical broadcast channel could be used for indicating radio problem detection. However, the quality of the common physical channel PBCH does not necessarily reflect the perceived quality of PDCCH and PDSCH.
The eNodeB will regularly, but not necessarily periodically, transmit uplink TPC commands that are addressed to a group of UEs. If a UE that belongs to a certain group has not detected such commands within a specified time interval, it could report radio problem detection to higher layers. The same concept is also applicable to other control formats that e.g. include time alignment commands, although these commands are sent in-band and are sent less frequently than the e.g. uplink TPC commands.
The eNodeB will regularly, but not necessarily periodically, transmit PCFICH which contains information on how many consecutive OFDM symbols of PDCCH that are sent within a sub-frame. There will be three known sequences of 32 bits sent via PCFICH to indicate either 1, or 2 or 3 OFDM symbols. As a quality metric of PCFICH, a UE can evaluate bit errors of the received sequences during a certain time interval and report radio problem detection when high number of consecutive bit errors exceeds a certain threshold.
Discontinuous Reception (DRX) in Connected Mode
The E-UTRA downlink also allows the possibility of discontinuous reception (DRX) in RRC_CONNECTED mode (or LTE_ACTIVE mode as commonly called). This permits UE to save its battery while stay connected since it would be required to wake up only at periodic instances according to the DRX cycle. The network can configure a DRX cycle between 2 ms and up to 2.56 seconds depending upon the type of service e.g. typically 2 or 4 ms for real time services such as voice over IP and 1.28 seconds for non-real time services such as browsing the Internet. During DRX the UE may temporarily go into continuous reception mode when the network is sending data. After the data reception the UE reverts to the normal DRX mode after a timeout configured by the network.
The UE will try to stay inactive as much as possible during the silent periods of the DRX cycle to achieve maximum possible saving of its battery. But this also implies that UE will mainly perform measurements at the wake up instances for mobility, radio link problem detection (out-of-sync detection, in-sync detection) etc. Due to insufficient measurement opportunities in DRX mode (depending upon the DRX cycle) the UE would be unable to promptly detect the radio link problem.
It is likely that a very large number of UE are kept in DRX mode. Secondly in DRX mode the network for transmitting data can abruptly switch the UE into continuous reception mode. The UE should therefore stay well connected in terms of radio link quality and so any radio link problem should be reported to the network promptly. Thus, the radio link problem detection should be designed to work effectively in both DRX and non DRX modes of operations.